StereoBM doesn't give proper output

edit

I want to find depth from stereo image. My code is working for one pair of an image but not working for another pair of an image.

Later, I want to convert video inputs from two cameras into a stereo output. I have tried that but that also have the same issue as the images (second pair of images) shown below.

I have tried it in with Java and Python and faced the same issue in both the languages.

Here is my java code

public static void main(String[] args)
{
    Mat left = Imgcodecs.imread("path", Imgcodecs.CV_LOAD_IMAGE_GRAYSCALE);
    Mat right = Imgcodecs.imread("path", Imgcodecs.CV_LOAD_IMAGE_GRAYSCALE);

    Core.normalize(left, left, 0, 255, NORM_MINMAX, CvType.CV_8U);
    Core.normalize(right, right, 0, 255, NORM_MINMAX, CvType.CV_8U);

    StereoBM bm = StereoBM.create(16, 15);

    Mat disparity = new Mat();
    bm.compute(left, right, disparity);
    ImageProcessor.showResult(disparity);
    mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
}

public static void showResult(Mat img)
{
    Imgproc.resize(img, img, new Size(640, 480));
    MatOfByte matOfByte = new MatOfByte();
    Imgcodecs.imencode(".jpg", img, matOfByte);
    byte[] byteArray = matOfByte.toArray();
    BufferedImage buffImage = null;
    try
    {
        InputStream in = new ByteArrayInputStream(byteArray);
        buffImage = ImageIO.read(in);
        JFrame frame = new JFrame();
        frame.getContentPane().add(new JLabel(new ImageIcon(buffImage)));
        frame.pack();
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setVisible(true);
    } catch (IOException e)
    {
        e.printStackTrace();
    }
}

How can I correct my code so that it will work for all images?

The below images gives expected output

The below images gives incorrect output